Field of the Invention
The present invention relates to a method for acquiring magnetic resonance data and an apparatus for implementing such a method.
Description of the Prior Art
Turbo Spin Echo (TSE) is a most important sequence for T2 weighted imaging in clinical application. The primary advantage compared to conventional spin echo technique is the reduced scan time. A TSE echo train consists of an excitation pulse followed by a train of refocusing pulses. The echo formed after each refocusing pulse is individually encoded so that multiple k-space lines can be sampled after the excitation pulse. TSE is one of many acronyms used for the technique. The most important others are fast spin echo (FSE), rapid acquisition with relaxation enhancement (RARE) and fast acquisition interleaved spin echo (FAISE).
Fat (lipid) signal appears bright in T2 weighted TSE imaging. The bright fat signal can obscure detection of lesions. A number of techniques are known to suppress the fat signal. The resonance frequency of protons bound to lipid molecules is approximately 3.3-3.5 parts per million (ppm) lower than the resonance frequency of protons bound to water. This fact can be utilized to suppress the bright fat signal. The most important clinical technique is still to use a frequency selective saturation or inversion pulses before each TSE excitation pulse. A saturation pulse excites the spins bound to lipids and leaves the spins bound to water unaffected. The fat signal is subsequently dephased with a spoiler gradient. The TSE echo train is executed immediately after the preparation pulse, i.e. before a significant number of fat spins being realigned with the static field due to T1 relaxation. Alternatively, the preparation module can use frequency selective inversion pulse. A certain time interval after the inversion pulse the lipid magnetization approaches zero since half of the spins have returned to the equilibrium state (again due to T1 relaxation). At this point in time the excitation pulse of the TSE echo train is executed. The disadvantage of selective lipid suppression or inversion is that these techniques rely on a homogeneous B0 field which often cannot be established in the entire imaging volume despite of shimming.
An alternative to spectral saturation or inversion is the use of a Dixon technique. The Dixon technique allows separating the fat and water component of the tissue into separate images. The Dixon technique can be used for fat suppression (diagnosis based on the water only images) or for fat water quantification were the local fat content of tissue is determined.
Inputs to the Dixon reconstruction are multiple complex images with different (and known) phase shift between water and fat component. The number of the input images needed and the required phase shift of these images depend on the particular Dixon technique. The classical 2-point Dixon technique, for example, requires two images, a first so called opposed phase image with phase shift of 180° between water and fat component and a second so called in-phase image with zero phase shift. Modern Dixon variants often require more than two input images and the desired phase shift increment between adjacent input images is often smaller than π (180°), e.g. 2π/3 in a 3-point Dixon technique.
One specific group of TSE Dixon sequences is important in the context of this invention. This group replaces the readout gradient in the middle between two adjacent refocusing pulses of a conventional TSE sequence by a train of readout gradients. The primarily advantage of this group is its motion insensitivity and comparable short scan time, as will be discussed later.
A gradient echo is formed whenever the net gradient moment is zero. The sequence is designed such that a gradient echo is formed during each readout gradient. This particular point is called the center of the readout (gradient). For all known TSE Dixon sequences which belong to the specified group the center of the readout coincides with the point of gravity of the readout gradient. If the center of a particular readout gradient lies half-way between the two adjacent refocusing pulses (i.e. coincides with the spin-echo) the phase shift between water and fat will be zero. The phase shift of another image depends on the temporal distance between the center of the corresponding readout gradient and the spin-echo point. The reason is that an off-center spin accumulates an additional phase which grows linear in time and is directly proportional to the off-center frequency. The different resonance frequency of the water and fat component therefore translates in a phase difference of the acquired images which depends (for a given B0 field strength) only on the temporal distance of the center of the corresponding readout and the spin-echo-point.
As stated before most Dixon reconstruction techniques require a particular specified phase difference Δϕ between water and fat and hence (for a given field strength) a particular temporal distance between the center of the readout and the spin-echo point. Within the specified group of TSE Dixon sequence the duration of a readout gradient is therefore limited by the temporal distance between the centers of readout gradients which belong to images with adjacent phase shift. Some Dixon techniques can cope with a range of phase differences between a minimum phase difference Δϕmin and maximum Δϕmax. In this case the maximum phase difference Δϕmax limits the duration of the readout gradient. By setting the specified phase difference Δϕ equal to Δϕmax the flexible Δϕ range can therefore be reduced to the fixed Δϕ case. The principle problem stays the same and a differentiation between the two cases is omitted in what follows.
The maximum resolution in readout direction is directly proportional to the 0th moment of the readout gradient. The maximum gradient strength and the maximum usable slew rate of the gradient system are limited and therefore also the resolution in readout direction. Further, since the sign of the gradient waveform in readout direction needs to be inverted at least once between adjacent readout gradients the maximum achievable gradient moment is much less than temporal distance between adjacent readout gradients times the maximum gradient strength, in general.
Common to most of the previous published TSE Dixon sequences is that corresponding k-space data of different echoes (i.e. with different phase shift between water and fat) are acquired after different excitation pulses.
This makes these TSE based Dixon techniques prone to motion that occur between excitations. TR times in FSE are comparatively long on the same time scale than the typical time intervals associated with physiological motion (due to breathing, heart beating or peristaltic). Further, B0 fluctuations (as a result of physiological motion or heating) between excitations leads to additional phase accumulation which cannot be distinguished from phase differences due to the fat-water frequency shift per se. Breath-holding is the most common technique to reduce breathing related artifacts. However, acquiring different echoes after different excitation pulses also increases approximately the minimum scan time by a factor n in an n-point Dixon technique. The reason is that the number of excitations and hence the number of TR intervals is increased by a factor n compared to a conventional non-Dixon scan. The resulting scan times exceeds the breath-hold capacities of most patients, at least for reasonable resolution. The following publications belong to this slow and motion sensitive group:    [1] Peter A. Hardy et al. “Separation of Fat and Water in Fast pin-Echo MR Imaging with the Three-Point Dixon Technique”. JMRI 1995; 5:181-185    [2] Jerzy Szumowski et al. “Double-Echo Three-Point-Dixon Method for Fat Suppression MRI”. MRM 34:120-124 (1995)    [3] Jingfei Ma et al. “Method for Efficient Fast Spin Echo Dixon Imaging”. Magnetic Resonance in Medicine 48:1021-1027 (2002)    [4] Weng Dehe et al. “Water Fat Separation with TSE BLADE Based on Three Points Dixon Technique”. ISMRM 2010, 2925    [5] Weiguo Zhang et al. “Separation of Water and Fat MR Images in a Single Scan at 0.35 T Using “Sandwich” Echoes”. JMRI 1996; 6:909-917    [6] Jingfei Ma et al. “Fast Spin-Echo Triple-Echo Dixon (fTED) Technique for Efficient T2-Weighted Water and Fat Imaging”. Magnetic Resonance in Medicine 58:103-109 (2007)    [7] Jingfei Ma et al. “A fast spin echo triple echo Dixon (fTED) technique for efficient T2-weighted water and fat imaging”. Proc. Intl. Soc. Mag. Reson. Med. 14 (2006) 3025
Reference 3 contains a TSE Dixon sequence with asymmetric readout. Opposite to the present invention it belongs to the slow, motion sensitive group which acquires different echoes after different excitations. Also the motivation is different. The intention of the asymmetric readout is not an increased resolution in readout direction but to avoid that the echo spacing is increased and that the number of slices that can be acquired in a given imaging time is reduced compared to a conventional TSE sequence. The increase in echo spacing and the reduced number of slices are problems of earlier TSE Dixon techniques. The efficiency statement (“number of slices in a given imaging time”) is only correct if the time for a single input image (echo) is considered. i.e. the TSE Dixon technique of Reference 3 needs n times the acquisition time of conventional scan for the n input images (echoes) of an n-point Dixon technique.
Acquiring different echoes (with different phase shift between water and fat) of a particular k-space line in a train of gradient echoes after a particular refocusing pulse greatly reduces the motion sensitivity without increasing the minimum scan time (measured in number of excitations (TR intervals)). The idea was first published by Zhang et al. for a conventional spin echo sequence on a 0.35 T scanner. In reference 5, Zhang et al. suggest to repeat the train of gradient echoes (termed “sandwich”) after each refocusing pulse in a TSE-like sequence. One discussed option was to acquire different k-space lines after different refocusing pulses to reduce scan times.
FIG. 1 shows a conventional TSE-Dixon sequence used by Zhang. In comparison with the well-known CPMG (Carr-Purcell-Meiboom-Gill) TSE sequence the single readout gradient is replaced by a train of three readout gradients with alternative sign. The center of the second readout gradient (with negative sign) coincides with the spin echo. The fat-water shift of the corresponding image is therefore zero. The center of the two other readout gradients deviates from the spin echo point by a time interval ΔTE chosen such that the phase shift between water and fat is −180° and +1800, respectively. The gradient in the readout direction between the 90° excitation pulse and the first refocusing gradient serves as prephasing gradient of the first readout gradient. The sign of this gradient and the sign of the first readout gradient after each refocusing pulse are the same since the refocusing pulse negates the phase of all spins. The second half of the first readout gradient (after the gradient echo) serves as prephasing gradient of the second readout gradient. Similar the second half of the second readout gradient serves as prephasing gradient of the third readout gradient. The second half of the third readout gradient has the same moment as the prephasing gradient between excitation pulse and first refocusing pulse. It therefore implicitly restores the dephasing of the spins so that it effectively unchanged by the combined action of the refocusing pulse and the three following readout gradients. The phase encoding axis is not shown in the FIG. 1. Phase encoding is performed before the first readout gradient and after the preceding refocusing pulse and is therefore identical for all three echo signals. Since a particular Fourier-encoding line of all images used for the Dixon reconstruction are acquired in the same echo spacing immediately after each other problems with patient motion are minimized. The dephasing due to the phase encoding gradient is undone after the third readout gradient and before the next refocusing pulse by a phase-encoding rephrasing gradient with same absolute moment but opposite sign.
Since the time ΔTE between adjacent gradient echoes decreases with increasing field strength (e.g. for a 180° phase shift, ΔTE=2.30 ms at 1.5 T and ΔTE=1.15 ms at 3 T) the technique can be applied with a spatial resolutions required for clinical imaging only at low fields. Even an extraordinary strong gradient hardware cannot solve this problem for a human scanner since the switching between readout gradients of opposite sign would induce nerve stimulations.